Optical communication technology having features of large capacity and high speed has been widely used hitherto for the implementation of a backbone network.
In recent years, as communication demand remarkably increases, the optical communication technology comes to be used for the implementation of an access network in an urban area, that is, a metro access network.
Furthermore, it has been started to apply the optical communication technology to data transmission (so-called datacom) between information processing apparatuses (servers, for example) which are installed in an identical room.
In such the metro access network and the datacom, a large number of optical devices (semiconductor laser, light receiver, optical amplifier, etc.) are used to implement the networks. Therefore, to advance the metro access network and the datacom, it is important to reduce the size and cost of modules on which such the optical devices are mounted.
The optical amplifier is an essential optical device to implement the optical network. As the optical amplifier, an optical fiber amplifier has been put into practical use, and is used in the backbone network. However, an optical fiber amplifier module, having a unified structure of the optical fiber amplifier with a control apparatus therefor, has a complicated configuration, which makes it difficult to achieve miniaturization and cost reduction.
In contrast, in regard to a semiconductor optical amplifier module having a unified structure of a semiconductor optical amplifier (hereafter abbreviated as SOA) with a control apparatus therefor, it is easy to achieve miniaturization and cost reduction.
Accordingly, the implementation of a next-generation access network and a datacom using the semiconductor optical amplifier module is under study.
As a way of using the optical amplifier in optical communication, there are two ways, namely, constant output control (auto power control; APC) and constant gain control (auto gain control; AGC).
The constant output control is a control method of the optical amplifier to output amplified light continuously having constant intensity, irrespective of variation in input light intensity. As the variation factors of the input light intensity, there are light source output variation and wavelength switchover.
The constant gain control is a control method of the optical amplifier to amplify input light with a continuously constant gain, irrespective of variation in the polarization direction and the wavelength of the input light. As the variation factor of the polarization direction of the input light, there is a state change of an optical fiber transmission path. Also, as the variation factor of the input light wavelength, there is a wavelength switchover at the light source, or the like.
In any control mode, desired control (APC or AGC) is achieved by adjusting the gain of the semiconductor optical amplifier synchronously with the variation of the input light.
The gain of the semiconductor optical amplifier varies with the magnitude of a current (drive current) supplied to the semi conductor optical amplifier. The larger the drive current is, the lager the optical gain becomes, while the smaller the drive current is, the smaller the optical gain becomes. By use of the above character, the gain of the semiconductor optical amplifier is adjusted, and thus the control like APC is achieved.
Additionally, in order to operate a semiconductor optical device such as the semiconductor optical amplifier and the semiconductor laser, it is necessary to control device temperature. Such the temperature control is carried out by a thermoelectric cooling device (peltier device), having the semiconductor optical device mounted thereon, or a heater mounted on the semiconductor optical device.                Patent document 1: Japanese Laid-open Patent Publication No. 2000-244458.        Patent document 2: Japanese Laid-open Patent Publication No. 2000-232251.        Patent document 3: Japanese Laid-open Patent Publication No. 2002-237784.        Patent document 4: Japanese Laid-open Patent Publication No. 2003-23208.        